1. Field of the Invention
This invention relates to a thermionic-electron-emitting or field-emitting material for use in appliances and equipment applying an electron beam. More particularly, it relates to an electron emissive material of ternary system hexaborides having the CaB.sub.6 type crystal structure, which is small in the work function and which is chemically stable.
2. Description of the Prior Art
In a variety of appliances and equipments applying an electron beam, there has recently been a strong request for an electron ray source attaining an electron beam of large capacity or an electronic device having a high brightness.
As a cathode material for emitting electrons, tungsten was exclusively used, whether the cathode was of the thermionic emission type or the field emission type. The reason for this use is that tungsten is very suitable for the purpose because it is low in vapor pressure, high in strength and excellent in workability. As the result of the recent advancement of apparatuses applying an electron beam, however, a material having higher degrees of characteristics than those of tungsten has come into demand.
In case of the thermionic emission, the current density J.sub.s (Amp./cm.sup.2) of an electron beam is represented by the Richardson-Dushman equation (1): EQU J.sub.s = A T.sup.2 exp (- .phi. /k.sub.B T) (1) where .phi. denotes the work function (eV), T the temperature (.degree.K) of the tip portion of the cathode at the electron emission, and k.sub.B the Boltzmann's constant. A is called the thermionic emission constant (Amp./cm.sup.2.K.sup.2), which is a constant sensitive to the state of the surface, especially to an adsorbed layer on the surface and the non-uniformity of the surface.
Assuming that A is fixed, it is understood from Equation (1) that as .phi. is smaller, the electronic current density J.sub.s becomes larger at low temperatures.
In case of the field emission, the electronic current density J.sub.s (Amp./cm.sup.2) is represented by the Fowler-Nordheim equation (2): ##EQU1## where E denotes the field strength (V/cm). a and b denote constants which do not involve the work function, and in the case of tungsten, they are known to be 6.2 .times. 10.sup.-.sup.6 and 6.8 .times. 10.sup.7, respectively.
In order to induce the field emission, an electric field in the order of at least 10.sup.6 V/cm is generally required. As apparent from Equation (2), even when the work function .phi. lowers slightly, the electronic current density J.sub.s remarkably increases even at low applied voltages.
Consequently, in both cases of the thermionic emission and the field emission, the employment of a cathode material of small work function is the first requisite for raising the electronic current density (and accordingly, the brightness).
Even when the work function is small, if the mechanical strength is low the cathode will be destroyed due to an electrostatic force or will be softened due to a high temperature. Therefore, a cathode material which has not only a small work function but also a high mechanical strength must be employed. Moreover, since the cathode is used in an ultrahigh vacuum, the life of the cathode becomes very short when a material of high vapor pressure is used. The cathode material need therefore be a stable material of low vapor pressure.
As cathode materiala fulfilling these requirements, it has hitherto been said that tungsten being a refractory metal and that borides having the calcium boride (CaB.sub.6) type crystal structure, especially lanthanum hexaboride (LaB.sub.6) are preferable.
The calcium hexaboride type crystal structure belongs to the space group O.sub.h.sup.1 -P.sub.m3m, and is a body-centered cubic lattice. Borides having this type of crystal structure are found among compounds which are obtained through reaction with alkaline-earth metals, rare-earth metals or some transition metals. Most of the borides have such very desirable properties for electron emissive materials, such that the melting point is high, that the vapor pressure is low, and that the hardness is great. Also, the borides are resistant against ion bombardment, and are apt to emit electrons by heat or electric field because the work function is low.
Since lanthanum hexaboride with a work function of about 2.66 eV was found to be suitable as the electron emissive material, 19 sorts of binary system hexaborides having the calcium hexaboride type crystal structure have been discovered. As ternary system or quaternary system hexaborides, (La, Na)B.sub.6 and (Sr.sub.0.4 La.sub.0.6)B.sub.6, for example, have been proposed in Zh Pkh. , Vol. 37, 1964, page 1872, by G. D. Samasonor et al. and in British Pat. No. 1,232,523.
However, although the ternary system or quaternary system hexaborides are lower in the work function than lanthanum hexaboride, they are smaller in the thermionic emission constant and are unsatisfactory in practical use. More specifically, the thermionic current densities of these borides at 1000.degree.C. are 0.1 - 0.01 Amp./cm.sup.2. The values are somewhat greater than in lanthanum hexaboride, but in the actual use, they are still insufficient and at least about 1 Amp./cm.sup.2 is necessary.
It has been known that, among the binary system hexaborides having the calcium hexaboride type structure, yttrium hexaboride (YB.sub.6) and gadolinium hexaboride (GdB.sub.6) are lower in the work function than lanthanum hexaboride and therefore suitable as a thermionic-electron-emitting material. The yttrium hexaboride and gadolinium hexaboride, however, are difficult to be prepared in the pure form, and are prone to be produced with other borides such as tetraborides, mixed therein. For example, yttrium hexaboride is prone to be produced under the mixed presence of yttrium tetraboride (YB.sub.4) and other yttrium borides. It is extremely hard to separate these other borides. The yttrium hexaboride phase which has a low work function and the yttrium tetraboride phase and the other yttrium boride phases which have high work functions are existent at the tip portion of the cathode in the mixed state, so that the current density of the electron emission and accordingly the brightness become unstable.
For such reasons, among the hexaborides having the calcium hexaboride type crystal structure, only lanthanum hexaboride has hitherto been barely permitted to be put into practical use, and the others have been unusable as stable cathode materials.